In one aspect the present invention relates to a method for the metered delivery of microbubbles for admixture with other components in the production of explosive compositions. In another aspect, the present invention relates to apparatus for handling and feeding microbubbles on a continuous basis which is highly accurate and can be employed to deliver microbubbles either on the basis of weight or on the basis of volume. In a further aspect the present invention relates to a control system for insuring that a deaerating holding vessel for microbubbles contains a constant volume thereof by providing for automatic delivery and shut off of microbubbles from a storage source thereof.
In the manufacture of explosive compositions which are based upon water in the form of oxidizing salt solutions, for example, it is well known in the art that the density of a composition plays a major role in the ultimate sensitivity of the explosive. Thus, in the past, occluded air has been employed in gel type explosives in order to attain the desired density. Recently however the use of glass or resin microbubbles to obtain desired densities of explosive compositions has gained wide acceptance in the explosives art. For example, it has recently been discovered that microbubbles, or other void containing materials, can be employed with a fuel component, an emulsifier and an oxidizing salt solution to form cap sensitive explosive emulsion products. Commercial manufacture of explosives employing closed cell void materials in the form of microbubbles entail the metered introduction of such materials for admixture with the other components of the explosive composition and because of the very low density of the microbubbles themselves the amounts which are added to known quantities of explosive compositions must be carefully controlled if the desired density of the final composition is to be achieved.
The microbubbles employed in explosive compositions can be produced from a variety of materials but they all are generally of a bubble or spherical shape and are hollow and either contain a gas such as air, or can be evacuated, or partially evacuated. In the preparation of cap sensitive explosive emulsion compositions the preferred types of microbubbles are discrete glass spheres having a particle size within the range of from about 10 to about 175 microns. In general, the bulk density of such particles can be in the range of about 0.10 to about 0.40 g/cc. Some preferred glass microbubbles which can be utilized in the preparation of cap sensitive explosive emulsions are the microbubbles sold by 3M Company and which have a particle size distribution in the range of from about 10 to about 160 microns, and a nominal size in the range of from about 60 to 70 microns, and densities in the range of from about 0.10 to about 0.4 g/cc. Other types of glass microbubbles are sold under the trade designation of Eccospheres by Emerson & Cumming, Inc., and generally have a particle size range from about 44 to about 175 microns and a density of about 0.15 to about 4.0 g/cc. Still other suitable microbubbles for use in cap sensitive explosive emulsions include the inorganic microspheres sold under the trade designation of Q-CEL by Philadelphia Quartz Company.
In addition to glass microbubbles, phenoformaldehyde microbubbles are available and can be utilized in the production of cap sensitive emulsion explosives. Further, microbubbles are available which are manufactured from saran. These saran microbubbles have a diameter of about 30 microns and a density of about 0.032 g/cc.
All of the above types of microbubbles are similar in that they have very small diameters and low bulk densities. The result is that the handling and accurate metering of such materials presents problems on a commercial scale. The physical characteristics of the microbubbles are such that an aerated quantity thereof has flow characteristics similar to Newtonian liquids, for example, water (under standard conditions). However, the handling characteristics of the microbubbles change drastically once they have become settled and deaerated so that handling them as liquids by the use of pumps or the like is not feasible. Furthermore, because of the low bulk density of the microbubbles, and their peculiar physical characteristics, precise measurement of a quantity thereof is difficult. For example, because of the highly particulate nature and low density of the microbubbles normal level indicators, such as floats and the like, which could be employed in a holding tank of liquid to determine the volume of liquid contained therein, cannot be successfully employed for the same purpose in a holding tank for microbubbles. Further, the very low bulk density of the microbubbles would require very sensitive weighing apparatus in order to determine the volume of microbubbles held in a holding tank by measuring variances in the total weight of the tank and microbubbles.
Because the accurate addition of known quantities of microbubbles to the other chemical constituents of explosive compositions in order to control the sensitivity thereof, as well as in other applications, is desirable, an automatic feeding system for the metered addition of microbubbles, having the difficult handling characteristics described above, would be advantageous. Further, a method for determining accurately the amount of microbubbles held in a holding tank thereof, for example, the hopper connected with a feeding apparatus, would also be highly advantageous.